"We were very interested in understanding whether fast food
prices might influence fast food behavior, so if prices were
high, did that reduce the number of visits to fast food
restaurants," senior author Penny Gordon-Larsen told Reuters
Health in an email.

Gordon-Larsen is a nutrition researcher with the Gillings
School of Global Public Health at the University of North
Carolina at Chapel Hill.

"We were surprised to see differential effects by
sociodemographic factors - such that lower socioeconomic status
individuals and blacks seemed to show stronger sensitivity to
prices," she said.

That suggests policies that affect fast food prices "might
have comparatively greater influence on the groups at highest
risk for diet-related chronic diseases," she added.

Most studies on this topic have been small or looked only at
one point in time, Gordon-Larsen said. For their report,
published in JAMA Internal Medicine, she and her colleagues
examined changes in fast food prices and health over two
decades.

They used data from a long-term study of 5,115 young adults
living in one of four large U.S. cities.

Participants reported on their fast food consumption and
underwent physical exams in 1985 when the study began and after
seven, 10, 15 and 20 years. A total of 3,537 of them completed
the study in 2006.

The researchers also tracked fast food price information
that included data on pizza, cheeseburgers and fried chicken
from national chains during that span. They adjusted prices for
inflation.

Black people and those with less education tended to eat
fast food more often than other consumers. But they were also
more sensitive to price increases.

For instance, when fast food cost $1.25 per serving, black
people ate it 2.20 times per week and white people ate it 1.55
times per week, on average.

When prices rose to $1.53 per serving, black people dropped
their fast food consumption to 1.86 times per week. White people
held relatively steady at 1.50 fast food meals per week.

The researchers also noticed a connection between fast food
prices and health. Lower prices were linked with heavier weights
among black and less-educated participants and higher rates of
insulin resistance (a precursor to diabetes) among less-educated
and middle-class participants.

"Fast food has been getting cheaper," Dr. Mitchell Katz, a
deputy editor for the journal, writes in an accompanying note.

"As the authors show, when fast food is cheap, people choose
it more often. Blacks and persons with less education were more
sensitive to fast food price, indicating that low cost fast food
may be contributing to disparities in obesity rates," he adds.